An outflow from a detention/retention facility that provides for the safe overflow of floodwaters for large storms that exceed the design capacity of the outlet or in the event of a malfunction. The emergency spillway prevents the water from overtopping the facility.

A shallow notch in the lagoon sidewall to allow lagoon overflow to spill over into a secondary containment or into a grass infiltration area in case of emergencies such as unexpectedly large rainfall event. The spillway should be designed and constucted under the supervision of a registered professional engineer or the NRCS and must be diverted away from water courses. While overflow should be avoided if at all possible, failure to provide a planned overflow in case of rainfall events that exceed design values risks berm failure that can result in a catastrophic pollution situation.